Cutting an elegant chamfer along the top corner of a hedge counteracts its natural habit of growing wider at the top than the base, and increases the surface area that receives direct sunlight.
By default, good hedges are grown with a “batter”, meaning that they are slightly wider at the base than the top. When a hedge gets overgrown, it will always lose its batter and become wider at the top.
Chamfers are usually added to old hedges when they start getting too big and/or wide at the top, losing their batter. But they might be by design from the start.
How to Cut a Chamfer Along the Top of Your Hedge
Beautifully demonstrated by Just Gardens on an overgrown green privet hedge.
One way to reduce the height of an overgrown hedge in stages is to increase the chamfer depth each year. That’s usually not necessary with vigorous privet, but it’s an option worth considering with slow growing Yew.